1. Field of the Invention
The present invention relates to sol-gel antireflective (AR) surface coating solutions and films, and methods for their preparation. Antireflective surface coating films reduce the reflectance of light from a surface (increase light transmittance through the film/substrate interface). More particularly, these sol gel coating films are single layer interference antireflective surfaces comprised of hydrolyzed polymeric metal alkoxides. These antireflective surface coating films are particularly useful for coating various plastic substrates to increase light transmission, without need to subject the film or the substrate to high temperature. Plastic substrates coated with the antireflective surface coating films of the present invention may be used, for example, in solar thermal and solar photovoltaic applications.
2. Discussion of Related Art
In solar applications, materials such as glasses and plastics have been used to minimize thermal losses, concentrate and focus light, and protect absorbers. Plastics, as compared with glasses, are less expensive, more easily formed into complex shapes, lighter in weight and not as brittle. Accordingly, the use of plastics, such as polycarbonates and acrylic polymers, is increasing in solar applications. Both glasses and plastics suffer from reflective losses at the material/air interface, the losses averaging about 7% for two surfaces. While numerous practical approaches to reducing the reflective losses for glass materials have been developed, few techniques are available for antireflecting plastics. Typical applications for plastics include fresnel focusing lenses in both thermal receivers and photovoltaic concentrator systems and glazing for flat plate collectors.
Sandia report SAND 84-0662 describes the development of a process for applying a single layer, antireflective coating to a Pyrex substrate for use in parabolic trough collector envelopes. The process comprises a preliminary step of aging the sol-gel in order to increase the size of the polymer contained in the solution, depositing the sol-gel onto the Pyrex substrate to form a film, heating the deposited film to 500.degree. C. and finally etching the heated film to produce the required refractive index and thickness for a single-layer interference anti-reflecting film on Pyrex for solar thermal applications. Due to the temperature limitation of plastics (maximum approximately 150.degree. C.) and because etching has been found to be ineffective at decreasing the index of an unheated sol-gel film, the process developed for Pyrex was discovered to be unsuitable for obtaining plastics having a low index, sol-gel, antireflective surface coating film deposited thereon.
British Patent No. 1,444,152, British Patent No. 1,530,833 and U.S. Pat. No. 3,996,067 describe standard techniques for depositing an AR film on plastics. The described techniques are vapor deposition, reactive plasma modification and fluorination processes. However, due to the drawbacks associated with these described techniques, none of the techniques have been used in large scale commercial solar applications.
The disadvantages of vapor deposition are: (1) large capital expenditure for deposition equipment, (2) possible temperature build up that can deform or melt the plastic surface, (3) restricted part size and geometry due to equipment limitations, and (4) separate coating processes are required for each surface.
Antireflection surfaces can also be formed by chemical modification of a surface by a reactive plasma. Again, this requires expensive equipment, possible heat build up, and size limitations.
Solutions containing fluorinated organics have been deposited on plastics which then exhibited antireflecting properties. However, fluorination processes appear to be limited to self developing photographic film applications and do not appear to be adaptable for large scale solar applications.
U.S. Pat. No. 2,466,119 to Moulton et al describes methods of depositing highly reflective and antireflective, multilayer films using (1) mixtures of SiO.sub.2 and TiO.sub.2 solutions, (2) in situ evaporation decomposition of titanium halides and alkyl silicates, and (3) control of reflectance/antireflectance by the number and sequence of films of different refractive indices. The methods described involve merely the reaction of the starting materials as opposed to polymer growth. Moulton et al had no concept of polymerization and the subsequent effect on refractive index. An attempt was made to prepare lower refractive index SiO.sub.2 films by the addition of MgCl.sub.2 to an alkyl silicate solution, followed by the selective leaching of Mg (a process analogous to current methods using etching of biphasic films to lower the refractive index by increasing porosity).
U.S. Pat. No. 4,286,024 to Yoldas describes methods of obtaining high temperature resistant aluminosilicate monoliths and protective films for quartz and alumina substrates. The methods described require treatment at very high temperatures (1200.degree. C.-1400.degree. C.) specifically to eliminate porosity in the monolith/film. These methods are unsuitable for coating plastic substrates which have a maximum temperature of 150.degree. C.
U.S. Pat. No. 4,361,598 to Yoldas describes the use of sol-gel techniques to deposit dense antireflective SiO.sub.2 /TiO.sub.2 films onto solar cells and stainless steel or silicon ribbon. The refractive index range attainable using mixtures of these solutions is 1.4-2.4. The refractive index required for AR film formation on plastics is .about.1.22 and cannot be achieved using the methods of Yoldas without the introduction of porosity. In addition, Yoldas' method requires heat treatments considerably higher than the upper temperature limits of plastics. Refractive index control is achieved by composition, firing temperature (300.degree. C.-600.degree. C.) and firing atmosphere. In contrast, the present invention achieves refractive index control totally in solution in order to deposit a film with an index of .about.1.22 and which requires no heat treatment.
U.S. Pat. No. 4,397,666 to Mishima et al primarily describes a method for depositing thick sol-gel derived films by using "viscosity adjusting agents". The refractive index of these films is not addressed and films obtained using this method range from 0.3 .mu.m-1.0 .mu.m thick. Such thicknesses are considerably greater than the 0.123 .mu.m thick film specified by Fresnel's equations for AR film formation on plastics.